There are conventional refrigeration systems of the type that have a refrigerant circuit which employs carbon dioxide as a refrigerant and performs a vapor compression refrigeration cycle using a supercritical cycle (see JP-A-2001-133058).
Such a prior art refrigeration system is provided with a refrigerant circuit including a lower stage compressor, a higher stage compressor, a heat dissipation side heat exchanger, a first pressure reduction unit, a gas-liquid separator, and a second pressure reduction unit which are connected sequentially, wherein gas refrigerant in the gas-liquid separator is directed to between the lower stage compressor and the higher stage compressor.
The aforesaid refrigeration system uses a supercritical cycle. Therefore, in the heat dissipation side heat exchanger, the refrigerant enters a supercritical state and no condensation temperature exists. Therefore, based on the temperature of refrigerant at the outlet of the heat dissipation side heat exchanger or based on the temperature of air around the heat dissipation side heat exchanger, the amount of pressure reduction by at least either one of the first and second pressure reduction units is controlled such that the pressure of high pressure refrigerant in the refrigerant circuit is optimized.